Ozone is a reactive triatomic allotrope of oxygen that has applications in chemical production, disinfection, drinking water treatment, air purification, bleaching of fabrics and wood pulp, wastewater treatment, and food processing. Most of the ozone used in these applications is produced by corona discharge systems using air or high-purity oxygen as the feed gas. Ozone also may be produced from air or oxygen by the action of ultraviolet light or by cold plasma generators.
High purity oxygen is used as the ozone generator feed gas in most large industrial applications of ozone. The conversion of oxygen into ozone in commercial corona discharge generators is typically between 4 and 13%, and in certain applications the resulting oxygen-ozone mixture is provided as product directly to the downstream user without further treatment. Because the cost of the unreacted oxygen is a major part of the ozone system operating cost, it is desirable in many situations to recover the oxygen from the oxygen-ozone mixture for recycling to the ozone generator. This can be accomplished, for example, by pressure swing adsorption (PSA) in which ozone is selectively adsorbed from the ozone generator outlet stream, and the recovered ozone-depleted oxygen is recycled to the ozone generator. The adsorbed ozone is desorbed by a sweep gas such as air or nitrogen, and the mixture of ozone and sweep gas is provided as product to the downstream user.
Ozone-oxygen PSA systems often use zeolite adsorbents for the selective adsorption of ozone from oxygen. It is known that zeolite adsorbents can promote the decomposition of ozone, and the degree of ozone decomposition can adversely affect ozone cost and increase the operating cost of the ozone-consuming process. The degree of ozone decomposition can be reduced by using a zeolite that contains pre-adsorbed components such as water, carbon dioxide, argon, or sulfur hexafluoride as described in U.S. Pat. No. 5,810,910. These components, which are non-reactive with ozone, are adsorbed on the adsorbent prior to ozone adsorption.
In ozone-oxygen PSA systems using cold plasma generators for the generation of ozone, nitrogen is often added to the oxygen feed gas for the purpose of stabilizing the plasma discharge. Unfortunately, the addition of nitrogen to the generation process often results in the formation of NOx, which can have several detrimental effects on the ozone-oxygen PSA system. In the presence of moisture, NOx will form nitric acid, which is highly corrosive and can shorten the service life of system components including, for example, ozone generator cathodes. In addition, NOx adsorbs onto many types of adsorbents used in oxygen-ozone PSA systems and can build up if the volume of gas used in a “sweep step” (to remove ozone from the adsorbent) is not sufficient to also remove adsorbed NOx. Adsorbed NOx also catalyzes ozone decomposition, which reduces the amount of ozone that can be recovered in a produced gas stream.
There is a need for an alternative means for improving the stability and efficiency for cold plasma ozone generation without forming undesirable by-products, such as NOx.